Here's how to create a Caste Shadow effect: PAIge

Crop or use the magic wand to delete the background to the picture.
Go to Layer: duplicate layer.
Go to the layer you just made and use free transform to shrink the picture to what you judge to be the right size.
Right click and click to distort to make the image to the angle you desire.
Go to Filter, Blur, Gaussian blur.
A box will have just popped up. Use the bottom adjustment bar to adjust the blur to the right thickness for the shadow (usually 4.7 depending on the lighting of the picture.).
Now look to the right of the screen and find the opacity of the image.
Adust to so that the shadow looks realistic (usually around 40%).
Now put the layer below the original and you are done.

*Sometimes you may need to turn the image black for a good looking shadow.